1. (Field of the Invention)
The present invention relates to a curable composition that can be favorably used as a dental composite restorative. More specifically, the invention relates to a curable composition which forms a cured product that features very high mechanical strength and, particularly, a very high fracture toughness value and a very high bending strength.
2. (Prior Art)
In recent years, a composite restorative has been much used as a restoring material in the dental field since it imparts a color equivalent to that of natural tooth and it can be easily handled. The restorative is generally used in a manner that a cavity in a tooth is directly filled with the restorative and is polymerized, and is cured by such a polymerization means as irradiation with light to restore the tooth. The restored part accomplished by the above-mentioned method has a relatively low mechanical strength. Therefore, this restoring method is generally effected for the portions where mechanical strength is not much required.
Recently, however, the composite restorative has been used even for restoring posterior teeth etc. that receive the pressure of occlusion. For instance, a mold of the cavity of a tooth is filled with the curable composition which is then polymerized upon the irradiation with light or by heating for a sufficient period of time, so that the composition is adhered to the cavity.
However, the restored portion obtained by polymerizing the curable composition according to the above-mentioned method, has a strength smaller than that of a natural tooth, and is broken at the edges due to a strong pressure of occlusion.
In order to solve this problem, it has been reported that a technology has been developed for obtaining a curable composition containing a filler at an increased density by using an inorganic powder having dissimilar particle sizes at a particular mixing ratio, or for blending a polymerizable monomer with irregular-shaped inorganic particles having particular particle diameters and properties and with spherical inorganic particles having particular particle diameters at particular ratios (Japanese Patent Laid-Open No. 132102/1991) and a method of obtaining a high mechanical strength by mixing spherical inorganic particles having particles of the order of submicrons and inorganic fine particles having particle diameters of from 0.01 to 0.1 .mu.m and further by using the highly dispersed mixture particles as a filler (Japanses Patent Laid-Open No. 12305/1996). The cured products obtained by these method and technology, however, have bending strengths which are smaller than 250 MPa. There has been no curable composition available that was capable of forming a cured product having a bending strength greater than 270 MPa possessed by the dentin of a tooth.
Besides, according to a report by Fujishima et al., "The Journal of the Japanese Society for Dental Materials and Devices, Vol. 13, No. 4, pp. 375-380, 1994", the breaking toughness value (K.sub.1C) which is an important property related to the breakage of the composite restorative is as low as 2.28 Mpa.multidot.m.sup.1/2 at the greatest.